Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated wireless access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated wireless access points communicate with each other directly, via a system controller, via the public switched telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency (IF) stages, and a power amplifier. The data modulation stage, also referred to herein as a baseband processor, converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more IF stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. A receiver of the radio transceiver performs the inverse of these operations to produce raw data.
Wireless Local Area Networks (WLANs) operate according to various operating standards, e.g., IEEE 802.11(a), IEEE 802.11(b), etc. Other standards are under development and/or are expected to be developed as communication systems evolve. Operations compliant with one of these operating standards ensure interoperability with equipment of other vendors that is also compliant with the operating standard. One aspect of many data communication standards relate to protecting data or facilitating the detection of data with error to trigger a specified corrective action. For example, forward error correction (FEC) is a technique used by a receiver or a receiver portion of a radio transceiver for correcting errors introduced into a signal while being transmitted over a communication channel without requiring retransmission of any information by the transmitter. In some protection schemes, forward error correction is used to protect the data within a portion of a frame, but is not used to protect all transmitted information. For example, in one system, FEC is used at the medium access control (MAC) layer to protect MAC layer signals. In this scheme, physical layer data, which includes important information required to process the data frame, is transmitted in a header without protection. Accordingly, if an error is introduced in physical layer, forward error correction may not be used to correct the physical layer information or data and may result in loss of the entire frame.
One particular limitation relating to forward error correction is known as scrambler error propagation. Typically, physical layer modulation schemes include the use of a pseudorandom sequence that is logically applied to a received bit stream of a data frame to achieve certain desired effects, including the avoidance of the generation of tones in the data resulting from specified data patterns in the transmitted data. For example, the bit stream may be exclusively OR'ed with the pseudorandom sequence. The pseudorandom sequence, however, must be initialized to work properly. Accordingly, either a scrambler initialization value for the pseudorandom sequence or a value that identifies the scrambler initialization value for the pseudorandom sequence is transmitted in a physical layer header and is used to initialize a scrambler to de-scramble a received data frame. In one embodiment, a value that identifies a scrambler initialization value is transmitted in a service field transmitted at the physical layer. For compatibility reasons, forward error correction is added in some networks at other layers above the physical layer. For example, in 802.11 standard based devices, forward error correction is added at the medium access control (MAC) layer rather than the physical layer. Thus, the physical layer header fields and the scrambler initialization field cannot be protected by FEC since they are not part of the MAC layer frame and are received and processed prior to the MAC layer frame. If the scrambler initialization field is received in error, the entire frame is corrupted. Thus, the benefits from utilizing forward error correction are limited only to errors that occur in the frame body and not to errors that occur at the physical layer header.
A need exists, therefore, for a solution to the scrambler error propagation problem and, more specifically, that allows for the proper scrambler recovery even if the transmitted scrambler initialization value is corrupted due to error during transmission.